1. Field of the Invention
This invention relates to an improved process for obtaining free flowing powders, and more particularly relates to a method of forming substantially spherical, dense particles from agglomerates of finely divided particulate material, and also relates to the resultant product.
2. Prior Art
Free flowing powders are useful in a variety of applications in the ceramic and metallurgical arts, such as in the formation of powder compacts, in casting and in coating operations, such as flame spraying.
Metallic and ceramic flame spray coatings are frequently applied to various articles to impart properties such as hardness, wear resistance, good lubricity, corrosion resistance, improved electrical properties or perhaps simply to build up a used part which has worn below usable tolerances.
Powders for flame spraying are desirably uniform in size and composition, and relatively free flowing. Flowability must be sufficient for the powders to be uniformly transported to and injected into the flame. In general, the finer the powders, the poorer the flow characteristics. Although considerable advances have been made in powder feeding equipment, powders less than about 40 micrometers generally do not flow well enough for general use.
The ceramics and powder metallurgy industry have used various agglomeration methods in order to make free flowing powders of normally non-flowing small diameter powder particles, usually involving use of an organic binder to promote formation of the agglomerates. Because of their larger sizes and relatively lower surface area the agglomerates have improved flow properties. Unfortunately, such agglomerated product also has a lower apparent density than the beginning particulate product. This property is the weight of a given volume of uncompacted, loose powder, and is important in flame spraying in that the weight of the coating being deposited depends on the weight of the volume of powder which the flame gum feeder will accept. In addition, the agglomerated product has a larger mean particle size than the beginning material. This is important in that when considering two materials of comparable size ranges, the one having the smaller mean particle size gives a denser, smoother coating. Strength is often improved with denser coatings and smoother coatings require less finishing by grinding or machining.
Flame spray powders having high apparent densities have been made by atomization of molten material. However, atomization processes are characterized by low yields of particles within the desired size range. Furthermore, powders of refractory material are difficult and costly to produce by atomization techniques primarily because of their high melting points.